A typical fuel pumping system for an aero-engine comprises a low pressure (LP) pumping stage operable to draw fuel from a fuel tank, supplying the fuel to the inlet of a high pressure (HP) pumping stage. The LP pumping stage typically comprises a centrifugal impeller pump whilst the HP pumping stage typically comprises a positive displacement pump in the form of a twin pinion gear pump. A jet pump is often incorporated into the inlet side of the LP pumping stage, the jet pump serving to reduce the ratio of vapour to liquid at the inlet to the LP pumping stage, too high a proportion of vapour at the inlet to the LP pumping stage adversely affecting the performance thereof, or resulting in de-priming thereof.
Where an aero-engine is upgraded part way through its working life, one change which is sometimes desired to be made is to replace the control system associated with the engine. Older engines typically incorporate hydro-mechanical control systems and it is desirable to be able to introduce electronic control systems, and in particular to incorporate a full authority digital engine controller (FADEC) when making such upgrades. The electronic engine controller (EEC) which forms part of a FADEC usually requires a dedicated electrical power supply. Often, the accessory gear boxes associated with older engines do not include sufficient drive output pads to permit the simple addition of an electrical generator typically in the form of a permanent magnet alternator (PMA) to provide power for the EEC, and instead the generator has to share one of the drive pads with, for example, part of the pumping system. Older fuel pumping systems are typically designed to operate at relatively low rotational speeds, and in order to avoid having to provide a large electrical generator designed to provide a sufficient output at such low operating speeds it is often necessary to provide a gear arrangement to permit the generator to be driven at higher speeds, and thus allow a smaller, lighter weight unit to be used.
The gear arrangements used in such applications need to be lubricated. Oil/air mist lubrication is effective, but can only be used where a separate lubricating oil supply is available. Where this is not possible, fuel is used as the lubricant. As the use of a fuel/air mist for lubrication purposes would carry an unacceptable high risk of explosion, the lubrication is achieved by having the gear arrangement fully submerged or drowned in fuel. Although this achieves lubrication in a safe manner, the rotation of the gears of the gear arrangement in fuel rather than air increases the resistance to rotation thereof. These so-called windage losses are proportional to the density of the fluid in which the gears are rotated and as fuel has a density approximately 1000 times that of air, it will be appreciated that significant windage losses are incurred.
In addition to being related to the density of the fluid in which the gear arrangement is rotated, windage losses are also related to the diameters of the gears used. The sizing and installation configuration of the HP pumping stage and PMA typically require large gears to be used to straddle the relatively large spacing between their axes of rotation, thus further increasing the windage losses.
U.S. Pat. No. 5,669,842 and GB2180022 describe air/oil lubricated drive schemes intended for use in automotive applications. U.S. Pat. No. 5,413,466 describes a drive scheme for a fuel pumping system in which windage losses are reduced.